1. Field of the Invention
The present invention relates a wave-filtering device, and particularly relates to a wave-filtering device automatically changing the frequency bandwidth in accordance with the different types of signals.
2. Description of the Prior Art
In the recent years, due to the advancement of technology, an integrated system has an increasingly improved function of processing video and voice data. For example, in the past, most of the processed voice data is human beings' voice signals, such as the voice on the phone. However, as the network technology advances, electrical music files, such as MP3, are prevailing. Therefore, the output unit of the integrated system has to process two kinds of signals requiring different frequency bandwidths. Some MP3 music files require high fidelity and have frequency bandwidth of 22 KHz. Nevertheless, due to the limitations of the telecommunication laws and regulations, the phone voice has a frequency bandwidth of 3 KHz.
Because noise is inevitably generated during the process of processing and transmitting the signals, the output unit of the integrated system generally has a wave filter for filtering out the unnecessary noise and keeping the voice signals remained. The frequency bandwidth of the wave filter is the criterion for determining which signals to be remained and which signals to be filtered out. However, because the difference between the frequency bandwidths of the MP3 music and phone voice is big, the output unit having only one wave filter with a fixed frequency bandwidth cannot implement the mentioned function.
In order to make the output unit successfully handle the signals with different frequency bandwidths, a digital signal processor (DSP) is provided for this purpose. The digital signal processor comprises a digital wave filter so that the DSP can obtain a digital signal within an appropriate range of frequency after transforming the signal by using Fast Fourier Transform (FFT), and then feed the obtained digital signal into a digital/analog (D/A) converter. Therefore, the D/A converter outputs a digital signal within an appropriate range of frequency. However, the DSP is expensive, and the DSP has to cooperate with software so that the spending on the software will increase. Besides, in the hardware, a memory with greater storage capacity is further required for storing the programs. In summary, using the DSP will greatly increase the cost of the integrated system without respect to hardware or software.
Besides using the DSP, another method is provided for enabling the output unit to successfully handle the signals with different frequency bandwidths. Two wave filters with different frequency bandwidths are installed in the output unit. The wave filter with smaller frequency bandwidth is used when outputting the phone voice, and the wave filter with greater frequency bandwidth is used when playing MP3 music files. However, the cost of using two wave filters is high.